Scientists bring us closer to wider use of phase-change memory (PRAM) chips.

By pre-organizing atoms in a bit of phase-change memory, information can be written in less than one nanosecond, the fastest for such memory. With write speeds comparable to the memory that powers our computers, phase change memory could one day help computers boot up instantly.

Phase-change memory stores information based on the organization of atoms in a material, often a mixture of germanium, antimony, and tellurium (Ge2Sb2Te5 or GST). A voltage pulse heats the metal and disordered atoms in the crystal rearrange into an ordered crystal. Restoring the disordered arrangement by melting the glassy material erases the information. A computer reads each bit by detecting the lower electrical resistance of the ordered crystal.

Micron sells small phase-change memory (PRAM) chips. Companies like IBM and Samsung are working on PRAM chips too.

Phase-change memory could one day replace flash memory in our cellphones, just as Samsung briefly tried in a commercial smartphone. PRAM can top the density and write times of flash memory. And like flash, PRAM is nonvolatile, meaning that it retains its information even when a device is powered off.

That makes phase-change memory an intriguing candidate to replace the volatile DRAM that powers our computers. But a computer that boots up instantly using PRAM is still a long way off, partly because the materials can’t be switched from disordered to ordered, or written quickly enough. Most phase-change materials crystallize slower than the 1-10 nanoseconds it takes to write a bit of DRAM. And materials that crystallize faster at PRAM operating temperatures tend to naturally organize at lower temperatures too, says Stephen Elliott of the University of Cambridge. Therefore, they slowly crystallize and erase themselves over time.

Elliott and his colleagues have boosted the crystallization time, and thus the write speed, of a stable PRAM bit. They pre-organized the atoms in a chunk of Ge2Sb2Te5 using a weak electric field. The scientists sandwiched a 50nm-wide cylinder of GST between two titanium electrodes and applied 0.3V of potential across the material. A 500-picosecond burst of 1V electrical potential triggered the crystallization, which is about 10 times faster than the best speed using a germanium-tellurium material.

The scientists melted the GST crystal, thus erasing the memory, with a similar 6.5V pulse. The material’s resistance was stable for 10,000 write-rewrite cycles.

Computer models of the material show that the constant low voltage causes tiny seed crystals to form, basically “priming” the atoms for complete crystallization.

Most researchers have tried to improve the switching speed of phase change memory by randomly inserting metals into GST, says Robert Simpson, at the Institute of Photonic Sciences in Spain. Learning how to create crystal seeds through simulations and then demonstrating how these seeds speed crystallization in a device is a more scientific approach to developing new memory materials, he adds.

Eric Pop, of the University of Illinois, Urbana-Champaign, is excited to see the speed limits of phase-change memory bits, but wonders if the extra power needed to maintain the low priming voltage would influence the speed and energy consumption of a chip containing many phase-change bits. Ultimately, consumer cost influences the commercial viability of PRAM chips, he adds.

This story was updated on June 25, 2012 to reflect the fact that GST is a glassy material, not a metal. References to voltage as energy and power were replaced by electrical potential.

I'm a bit confused, how fast did the crystals actually form? Did it happen in the 500 picoseconds mentioned in the article? That seems unlikely as that would be faster than DRAM. If it was that fast, they could have done a whole lot more than 10,000 rewrite cycles for their test--it would have only taken 5 microseconds!

My understanding of Phase Change memory is that it's not a candidate for replacing DRAM, but instead for replacing Flash memory. It just lives in the wrong order of magnitude speed wise to ever be a proper DRAM replacement.

So, do you need a very expensive and rare material like titanium to make these? That's going to get really expensive.

Titanium is one of the most abundant metals on earth. It's in your white paint and sunscreen among other things (in the form of Titanium Dioxide). The reason Titanium equipment costs so much is because it has to be melted and forged in a vacuum or inert gas, because any exposure to Oxygen will cause immediate combustion.

This sounds like the perfect replacement for NAND Flash, not for regular RAM (SDRAM): non-volatile, a little bit slower than DRAM, faster than a spinning magnetic disc and, unlike Flash, able to be addressed and securely erased / rewritten at the single bit level (maybe?). Sadly, the 10,000 cycles comment makes it sound like this material would have the same auto-decay properties that make Flash rely on strange write techniques. Also, the reliance on many different rare earth metals means that this will be too expensive to be workable in a decade or so.

So, do you need a very expensive and rare material like titanium to make these? That's going to get really expensive.

Titanium is one of the most abundant metals on earth. It's in your white paint and sunscreen among other things (in the form of Titanium Dioxide). The reason Titanium equipment costs so much is because it has to be melted and forged in a vacuum or inert gas, because any exposure to Oxygen will cause immediate combustion.

Thats overstating it a bit. Titanium is harder to work with on a number of fronts, and the raw material cost is higher because there isn't yet a viable continuous process for reducing titanium. I've seen some work by DARPA that may potentially bring the raw material cost of titanium metal down to the cost of aluminum. Still titanium will be harder to work with, mainly due to the fact that it is inherently more resistant to deformation (especially in its unalloyed hexagonal structure), but it is not that much more reactive than aluminum and it forms a protective oxide (rutile) which adds its own headaches.

I'd be more worried about the tellurium, which is about as abundant as platinum in the earth's crust. Still, for the very small amount of material that goes into this stuff, the raw material costs are probably negligible.

PCM (as it's most commonly called in the architecture community) is still viewed as a viable "replacement" for DRAM by architects (the guys who really have to care about its limited number of write cycles). It's always talked about in the context of "PCM that backs up a DRAM cache." So a future system that utilizes PCM will have, say, 32 GB of DRAM, backed up by 2 TB of PCM (numbers picked arbitrarily for a future where 32 GB DRAM is considered "small"). So instead of really being a direct replacement for DRAM, it's viewed as an additional level in the memory hierarchy after DRAM but before disk. In this context PCM is the "real" main memory, and the DRAM is just a cache. This, along with other recent work by architects to get the most out of available write cycles, make the concern about the write cycle limit pretty much disappear. I don't think anyone's really worried about that at this point.

As for it being non-volatile ... well, that's not exactly true. PCMs represent binary values by forcing the chalcogenide material into a particular crystalline/amorphous/hybrid state, which has a particular resistance. You write a bit by cooking the material to the particular resistance you want, and you read a bit by measuring its resistance. The problem is that the chalcogenide material naturally drifts towards a higher resistance state if you just let it sit there. This probably won't cause problems on the time scale of a few seconds, but it may be a problem on the time scale of days and years.

What struck me as funny is that of the three metals mentioned aside from titanium, only antimony is less expensive, and all three are orders of magnitude less common, yet Ti was the one he choose to worry about.

It sounded like the titanium was just used for the electrodes that created the electrical field. It wasn't clear from the article whether it absolutely had to be Ti because of some crucial physical property, or whether it was just used for convenience in this particular experimental rig. It could be that some more conventional material (like copper) could be substituted in mass-produced devices.

At this point I'd just be happy for a suitably fast and reliable Flash replacement. Flash is slowly getting to the end of its technology life due to reduced cycle life because of the shrinking manufacturing process (50nm = 10,000 cycles, 30nm = 5,000 cycles, 20nm = 3,000 cycles, you see where this is going). So having something that could come in and replace it with faster speeds and higher density is welcome.

In this context PCM is the "real" main memory, and the DRAM is just a cache.

Typically, you address a DRAM as a pointer and a NAND typically as a file handle from a sw point of view. It sounds like you are pretty sure the software interface for PCM is going to be a pointer. That implies PCM won't be able to support "bad blocks" that a file system maps around like NAND does. I think you are saying that a PCM (or PRAM?) will have NAND density, but DRAM repair levels.

Would you agree that the new memory model for servers is:L1/L2/L3 CacheDRAM (DDR4)PCM (? interface)<--- file system starts hereNAND - SSD (PCIe or SATA to ONFI3)HDD (SATA)Cloud

The new memory model for mobile is:L1/L2 CacheDRAM (LPDDR3)PCM (?) or NAND (?)Cloud

What struck me as funny is that of the three metals mentioned aside from titanium, only antimony is less expensive, and all three are orders of magnitude less common, yet Ti was the one he choose to worry about.

More like that was what he chose to highlight of the 4, rather than being the only one he worried about.

Volts are not a measure of energy. Please do not perpetuate this misinformation."applied 0.3V of energy": just leave off 'of energy' "pulse of 6.5V of energy": better would be "6.5V pulse"To know the energy delivered requires also knowing the time of the pulse and resistance of the material.

So, do you need a very expensive and rare material like titanium to make these? That's going to get really expensive.

Titanium is one of the most abundant metals on earth. It's in your white paint and sunscreen among other things (in the form of Titanium Dioxide). The reason Titanium equipment costs so much is because it has to be melted and forged in a vacuum or inert gas, because any exposure to Oxygen will cause immediate combustion.

Thats overstating it a bit. Titanium is harder to work with on a number of fronts, and the raw material cost is higher because there isn't yet a viable continuous process for reducing titanium. I've seen some work by DARPA that may potentially bring the raw material cost of titanium metal down to the cost of aluminum. Still titanium will be harder to work with, mainly due to the fact that it is inherently more resistant to deformation (especially in its unalloyed hexagonal structure), but it is not that much more reactive than aluminum and it forms a protective oxide (rutile) which adds its own headaches.

I'd be more worried about the tellurium, which is about as abundant as platinum in the earth's crust. Still, for the very small amount of material that goes into this stuff, the raw material costs are probably negligible.

From a semiconductor manufacturing standpoint, titanium is dirt cheap and not at all difficult to work with. Making Ti electrodes on a semiconductor device is a trivial task.

This article is a bit confusing. DRAM read/write speeds are 1-10ns? How long is PCM/PRAM? What was it before?

Quote:

A computer reads each bit by detecting the lower electrical resistance of the ordered crystal.

The processor doesn't do that reading, the electrical reading is done on the same die as the PRAM bitcells. This is similar how DRAM bitcells are read/sensed on the same die as the bitcells.

PCM/PRAM would have some serious security implications - what if the computer gets shut off and your passwords are still in memory? This doesnt work (well) now because DRAM doesnt last very long without electricity, although there was one paper where liquid nitrogen was able to extend the time to minutes.

Yeah, those theoretical attacks on DRAM where you find a running computer, rip the side off, freeze the memory and immediately stick it in some reader weren't something I was terribly concerned about. I'm sure I'll see it in a movie at some point, but in the real world the circumstances that would cause that attack to make sense are very extreme.

In this context PCM is the "real" main memory, and the DRAM is just a cache.

Typically, you address a DRAM as a pointer and a NAND typically as a file handle from a sw point of view. It sounds like you are pretty sure the software interface for PCM is going to be a pointer. That implies PCM won't be able to support "bad blocks" that a file system maps around like NAND does. I think you are saying that a PCM (or PRAM?) will have NAND density, but DRAM repair levels.

Would you agree that the new memory model for servers is:L1/L2/L3 CacheDRAM (DDR4)PCM (? interface)<--- file system starts hereNAND - SSD (PCIe or SATA to ONFI3)HDD (SATA)Cloud

The new memory model for mobile is:L1/L2 CacheDRAM (LPDDR3)PCM (?) or NAND (?)Cloud

Academic architecture papers assume pretty much the same interface for PCM as for DRAM (DIMMs on memory channels), with either a RAS/CAS type interface or perhaps a more packet-based interface. In either case, it's a memory address- (or pointer-) based interface and not a file-based interface. I've heard some industry people think this is funny, but that's still what academic architects are assuming. I think the hope is that NAND on PCIe will become unnecessary from a performance or capacity perspective, but NAND might still live on as permanent, realiable, nonvolatile storage (because PCM has that pesky resistance drift problem). If PCMs can be big enough, then maybe NAND will become completely redundant, and PCM will just be backed by magnetic disks.

I kind of doubt PCM's impact in the mobile space. I would rather have my DRAM on my mobile device be backed up by flash. I know this is never a wise thing to say, but how much RAM could a mobile device really need? Enough to warrant the use of PCM, or is DRAM's density enough? Who knows.

What struck me as funny is that of the three metals mentioned aside from titanium, only antimony is less expensive, and all three are orders of magnitude less common, yet Ti was the one he choose to worry about.

More like that was what he chose to highlight of the 4, rather than being the only one he worried about.

Could simply be an artifact of titanium being the last one mentioned.

My point was: that's like seeing a BoM including gold, silver, copper, and iron, and saying "You sure you want to use iron? That shit's expensive." Plus, none of the four are rare earths, nor are they particularly uncommon or expensive.

I´d like to see a computer that uses DRAM starting at the lowest address on the CPU´s address space and PCM starting at the highest address space, and use PCM to build an expandable ramdrive directly from firmware. This way one can have file-based storage at RAM speed.This architecture can even be implemented using battery backed DRAM instead of PCM, provided DRAM becomes less expensive.

What I don't understand is - why is it that in every single article about a new type memory or hard drive, people mention fast/instantaneous boot-ups as a pie in the sky. Is anyone really bothered by modern boot times? I usually keep my desktop PC in sleep mode, and it wakes from that in about 2 seconds. Even when I have to reboot, it takes about a minute, including all the various things running at startup.

What I don't understand is - why is it that in every single article about a new type memory or hard drive, people mention fast/instantaneous boot-ups as a pie in the sky. Is anyone really bothered by modern boot times? I usually keep my desktop PC in sleep mode, and it wakes from that in about 2 seconds. Even when I have to reboot, it takes about a minute, including all the various things running at startup.

So, really, is this still an issue for anyone?

It has important implications for hibernate and similar shutdown models. Currently hibernation essentially dumps the ram to the disk and then tells the kernel that when it next boots, load from the dump. Sleep is just a low-power mode that allows a computer to power-down most components, but everything is still in memory. PRAM could bridge a gap, or more likely merge the two. Basically the computer would just power off with all the data in PRAM, power on would use the PRAM like sleep, but no power usage like with hibernate. Obviously the advantages aren't in desktops, they are in laptops and mobile devices, probably tablets. Basically this means that you can just close the laptop and it will use on the order of microamps of power, effectively allowing you to keep it like that for days. Then when you turn it back on, it will almost instantly go back to where you were before.

What I don't understand is - why is it that in every single article about a new type memory or hard drive, people mention fast/instantaneous boot-ups as a pie in the sky. Is anyone really bothered by modern boot times? I usually keep my desktop PC in sleep mode, and it wakes from that in about 2 seconds. Even when I have to reboot, it takes about a minute, including all the various things running at startup.

So, really, is this still an issue for anyone?

It has important implications for hibernate and similar shutdown models. Currently hibernation essentially dumps the ram to the disk and then tells the kernel that when it next boots, load from the dump. Sleep is just a low-power mode that allows a computer to power-down most components, but everything is still in memory. PRAM could bridge a gap, or more likely merge the two. Basically the computer would just power off with all the data in PRAM, power on would use the PRAM like sleep, but no power usage like with hibernate. Obviously the advantages aren't in desktops, they are in laptops and mobile devices, probably tablets. Basically this means that you can just close the laptop and it will use on the order of microamps of power, effectively allowing you to keep it like that for days. Then when you turn it back on, it will almost instantly go back to where you were before.

Right, I understand the differences in power consumption, but modern laptops/tablets really don't consume that much power in sleep mode - I rarely bother shutting down my laptop, only using sleep (for days), and have no real issue with it.I get the interesting applications, and certainly the scientific interest, I just don't understand this preoccupation with fast boot times, when boot-up doesn't take that long anyway, and sleep has become a really useful option (unlike the old days, when Windows would go haywire when it tried to sleep or hibernate).

It's getting 10 thousand read/write cycles without deterioration *now*, as a lab experiment. Some devices out there being sold now incorporate flash memory that performs no better, or even worse. (The Stellaris C5 and Firestorm A2 series microcontrollers shipped with flash only rated for 100 cycles.) Don't assume devices eventually produced for personal computers or portable devices will have the same limitation.

As for software, file handles are one high level abstraction that is used in some specific systems, it's got nothing to do with the hardware. Many systems consider flash to be just another memory space, or just another segment of a memory space shared with static RAM, dynamic RAM, and various peripherals. PCRAM could be a drop-in replacement for both DRAM and NAND flash (or NOR flash, which it sounds closer to), depending on what the parts were designed for. Or it could be both, with the virtual memory system marking pages of memory for use as working memory, with the same memory being exposed as a filesystem device for bulk storage and addressed directly as RAM.